The free electron laser, or FEL, is a unique laser device in which a beam of electrons is deflected by a linear array of electrical or magnetic fields of alternately opposite polarities. The array, aptly referred to in the art as a "wiggler," imparts an undulating movement to the electron beam which causes emission of laser radiation. This can be used directly or can be applied to an existing laser beam to amplify it. A radio frequency FEL, or RF FEL, both generates a laser beam and amplifies it within an optical cavity. The RF FEL is suited to operate at very high powers, and can operate more efficiently than chemical or excimer lasers. It also has the potential for wavelength tunability. (In the remainder of this discussion the term FEL is intended to mean radio frequency FEL.) U.S. Pat. No. 4,287,488 issued on Sept. 1, 1981 to Brau et al. gives an overview of the theory of the RF FEL, and is incorporated herein by reference.
To realize the full potential of the RF FEL in both commercial and military applications, lasing at sideband wavelengths must be suppressed. Suppressing the sideband wavelengths is estimated to increase the electrical to optical efficiency by a factor of two to five. If a sideband suppressor were tunable, this would further allow tuning the FEL wavelength in real time.
Prior attempts at sideband suppression have involved narrow bandpass or sharp cutoff multilayer dielectric coatings on elements which attenuate unwanted sideband wavelengths. But this approach does not provide real time tunability over an appreciable wavelength range. While small variations in wavelength can be accomplished by varying the incident angle of the beam on the element, large wavelength variations necessitate the replacing of the element with one of different optical characteristics. This could take hours, or even days.
Some presently used devices place the dielectric coating on a transmissive element to couple energy out of the FEL. But this limits power and run time because appropriate transmissive materials are poor thermal conductors, and thus do not cool as needed. This is especially true at short wavelengths. Other designs would place the multilayer dielectric coatings on one of the resonator mirrors, but this approach limits the range of tunability.
One possible approach to sideband suppression is the use of a single grating to introduce an angular dispersion of the sideband. However, a grating which creates sufficient angular dispersion to use for outcoupling also strongly degrades or even destroys the performance of the FEL at the desired frequency. This is due to the broadening of the focus of the main line and also the deflection of the main line's position. This reduces laser intensity in the wiggler and therefore the effective energy transfer from the free electrons to the laser photons at the desired wavelength. Outcoupling also becomes problematic with a low dispersion that does not degrade laser performance, since it is difficult to achieve sufficient physical separation between the zero and first order within a reasonable distance. This alternative for sideband suppression thus introduces other problems into the performance of the FEL.